(1) Field of the Invention
This invention relates to a primer composition, especially to an aqueous primer composition which is applied previously to the application of finish coating on the surfaces of polyolefin resin substrates such as bumpers, side body moldings, instrument panels of automobiles and so forth. The polyolefins includes polyethylene, polypropylene, ethylene-propylene copolymers and the like which have no polar groups.
(2) Description of Prior Art
Because the olefin resin consists of non-polar elements of hydrogen and carbon, when an ordinary paint is directly applied, the adhesiveness of coated film is not good. Accordingly, in the conventional art, the surfaces of molded articles are subjected to chemical treatment or plasma treatment or they are coated by a primer coating mainly comprising polyurethane resin, fatty acid-modified polyester resin, maleic anhydride-grafted product of polypropylene or ethylene-.alpha.-olefin copolymer, chlorinated polyolefin, or acid-modified chlorinated polyolefin, and after that, application of finish coating is carried out.
Especially, in the method to apply a modified polyolefin primer mainly comprising grafted polymer of polypropylene or ethylene-.alpha.-olefin copolymer with maleic anhydride, chlorinated polyolefin, or acid-modified chlorinated polyolefin, it is characterized in that the application of finish coating can be done without impairing the low temperature shock resistance of the polyolefin resin material.
For this reason, various kinds of primer compositions have been proposed, for example, in U.S. Pat. Nos. 4,299,754; 4,755,553; 4,647,365; 4,692,357; 4,883,834 and 4,874,818.
With regard to the above-mentioned primer compositions, there are problems of flammability and working environment because they are generally used as solutions in organic solvents. Accordingly, if these primer compositions can be used as solutions or dispersions in water, it is very advantageous in view of handling as compared with the solvent types.
It is difficult, however, to use these primer compositions in an aqueous dispersion type or aqueous solution type. Furthermore, even when they can be dispersed in water, such an aqueous primer cannot be practically used because the dispersion of pigment is not good and the application on polyolefin resin is difficult.
Still further, many kinds of water-soluble high polymer compounds and compositions have been hitherto proposed as adhesives and coating materials. However, it is difficult to apply these compositions intact on polyolefin resin. Even when they can be used for coating, the obtained coating film is not satisfactory in the adhesiveness to polyolefin resin, physical properties and water resistance, so that there has been no practically acceptable primer compositions.
For example, emulsions of ethylene polymer and styrene polymer are used for preparing adhesives and coating materials. They are mainly used for goods made of paper, wood or metals, however, any application, especially as a primer, for polyolefin resin has never been proposed.
It is considered that, because the wetting tension of polyolefin resin is generally low, when an aqueous solution or dispersion having a high surface tension is applied to it, any desirable coating film cannot be obtained owing to cratering or other defects.
Furthermore, because these aqueous compositions of the emulsion of ethylene polymer or styrene polymer are usually prepared by using emulsifying agents, the agent remains in the formed coating films on polyolefin substrates, and the agent impairs not only the physical properties of the coating films but also the water resistance of the coating film.
Still further, proposed in U.S. Pat. No. 4,174,335 is a method to prepare aqueous dispersion using a specific modified olefin polymer without any surface active agent. In U.S. Pat. No. 4,775,713 and European Laid-Open Patent No. 183,493, carboxylic acidmodified olefin polymers having specific properties are proposed as auxiliary dispersing agents for aqueous dispersions. However, when the content of the introduced carboxylic acid is small, the dispersibility is not good enough and it is substantially difficult to disperse pigments simultaneously. On the other hand, when the content of the introduced carboxylic acid is too large, the solvent resistance and water resistance of obtained coating film are not satisfactory.
As described above, there are problems of flammability and working environment because the primer compositions for polyolefin resin are generally used as solutions in organic solvents. If these primer compositions can be used as solutions or dispersions in water, it is very convenient in handling as compared with the solvent type compositions.
Many aqueous compositions has been hitherto proposed as adhesives and coating materials. However, when they are applied to polyolefin resin substrates, they are difficultly applied and obtained coating films are not satisfactory.
Furthermore, in the case that a large quantity of surface active agent is added to a primer composition in order to improve the coating property to polyolefin resin substrates, several properties such as water resistance and solvent resistance which are required for the primer coating cannot be made satisfactory.